The current invention relates to the field of medical anesthesia. More particularly it relates to the field of electronic monitoring of a patient undergoing anesthesia, especially for use during and after surgical operations. The invention more specifically relates to an electronic subsystem of the instrument used to monitor a patient""s state of awareness, more specifically still to the subsystem whereby electroencephalograph signals are reliably acquired from one or more electrodes attached to the patient""s head.
Traditionally in the administration of anesthesia it has been the practice for an anesthesiologist to use only clinical signs from the patient to estimate the depth of the patient""s anesthesia before and during surgical procedures requiring anesthesia. In recent years, however, it has become possible and practicable to manipulate certain transduced bodily signals, in particular electro-encephalographic (EEG) signals, to produce an indication of how anesthetized or alternatively how awake a patient is.
The crude EEG signals are acquired via gel or other conducting electrodes attached to one or more predetermined standard locations on the patient""s head. A modular system will then have a module for collecting and transmitting such signals to an analysis unit. Such a module is intended not only to assure that the actual electrodes attached to the patient""s head form a separate and potentially non-reusable module themselves but also to assure that the signals sent to the analysis unit are representative of the electrical activity in the patient""s head and not of the ambient electrical activity in the place where the system is being used, in most cases an operating room.
The operating room (OR) in a typical hospital is a particularly harsh electromagnetic environment for patient electronic monitoring, especially for EEG signals. The OR signal acquisition environment exacerbates conditions that minimize the signal-to-noise ratio of acquired EEG data. The most significant source of OR noise in the recorded data is the electro-cautery device commonly known as xe2x80x9cthe BOVIxe2x80x9d.
The BOVI has operating frequencies from 0.5 MHz to 2 MHz. Open circuit voltages of up to 3000 volts are drawn down during cutting when the device delivers up to 300 watts into a 100 xcexa9 load. This cauterizing discharge produces a large amplitude modulated RF signal, which couples to the EEG pre-amplifier through the signal leads and the preamplifier enclosure. Coupling modalities include direct radiation of the EM field to the patient-connected lead wires and coupling of the EM field to the pre-amplifier circuitry inside the shielded enclosure. Leadwire coupled radiation introduces artifact into the amplifiers common mode and normal mode signal pathways.
Since the BOVI generates noise well above the 0.5 to 100 Hz EEG frequency band, it would superficially seem that the BOVI should not be a problem. In practice this is not the case. Prior art EEG monitoring equipment displays substantial electromagnetic artifact during cautery operation. The prior art amplifiers saturate or block for the period that the BOVI is in use plus up to an additional minute while the high pass filter elements (0.1-0.5 Hz typical) recover from significant BOVI induced offsets.
In order to understand how the BOVI corrupts the EEG signal, we must first understand what is actually happening during its use. When the BOVI is first switched on, a very large transient is produced followed by steady state BOVI EM field. This is the case when the BOVI is not cutting. Most EEG amplifiers will display the turn-on transient of the BOVI and then settle down with little or no artifact present. When cutting starts, however, the 0.5-2 MHz BOVI signal is amplitude modulated at greater than 75%, during tissue ablation, with frequency components in the EEG passband and corresponding to the sampling frequency and its harmonics. Depending on input filter characteristics, these very large out of band signals leak through the passive input filter stage and a significant signal is present at the input of the pre-amplifier. Typical EEG amplifiers do not respond linearly to the presence of these high frequencies. More specifically, their slew rates are different in the positive and negative direction. They act much the same way that the detector does in an AM radio, stripping out the carrier and leaving the carrier envelope. In this case, the carrier envelope contains energy in a broad range of frequencies associated with the BOVI during the ablation of tissue, some of which lies in the EEG passband and some of which ends up in the passband as a result of aliasing.
In addition, it is not sufficient to be somewhat more resistant to BOVI. BOVI artifact reduced to below the threshold of the artifact detectors will nevertheless corrupt the EEG signal and its processed results. The improvement must be substantial, such that BOVI does not influence a computed EEG index or parameter. Residual artifact after only a modest improvement will either 1) increase the latency of the EEG index when detected and rejected, or 2) increase the signal variability and the unreliability of the EEG index when not detected.
A generally accepted method for circuit protection necessary to meet IEC601-2-26 includes the use of gas filled spark gaps, which shunt and dissipate most of the energy that would threaten preamplifier integrity. This approach has the limitation that it requires additional circuitry for current limiting and signal recovery to be placed between the shunt and the preamplifiers input circuitry. This is necessary since this type of shunt limits voltages to 50 volts or greater which can significantly extend amplifier signal recovery time and still cause permanent damage to the amplifier. The added circuit complexity and area increases the physical size, cost and exposure to electromagnetic fields.
There has thus been demonstrated a need for an economical device for preventing the corruption of EEG signals to be used for anesthesia and other medical monitoring by electro-cautery and defibrillator devices. It is the principal object of the current invention to provide such a system.
The patient module that is the current invention is an 8 channel EEG pre-amplifier whose signal acquisition and processing characteristics are optimized for use in the operating room and intensive care unit (ICU). This preamplifier uses superior techniques to suppress EMI and thereby virtually eliminates BOVI and other artifacts. This elimination has been demonstrated experimentally. The acquired signals will be transformed and analyzed targeting a variety of spectral and temporal properties to measure the patient""s level of awareness. The frequency band of interest is from 0.5 Hz to 100 Hz and the dynamic range of the amplifiers is from 0.25 xcexcVolts to 1400 xcexcVolts. The patient module includes the following features essential for superior OR and ICU signal acquisition performance:
1) Optimized multistage input filter
2) Optimized input stage circuit topography
3) Ultra-isolation
4) Oversampling
5) Multiplexer inter-sample charge dump
6) High Performance, low frequency enhanced shielding.